130 THE APODOTJS HOLOTHURIANS 



TBOCHODBBMA ELEGANS. 



PLATE VIII, FIGS. 7-14. 



Trochoderma elegans Theel, 1877, p. 11 ; pi. n. 



LENGTH. 10-15 mm. 



COLOR. Silvery white. 



DISTBIBUTION. Beported from Nova Zembla and Sea of Kara (Theel et al.). 



REMARKS. The geographical distribution of this species is somewhat doubt- 

 ful, owing to the unfortunate error of Stuxberg (see Ostergren :03, p. 16, foot- 

 note) in confusing Myriotrochus rinkii with it. It lives in comparatively shal- 

 low water, 9-220 m. 



ACANTHOTHOCHUS Danielssen and Koren, 1879. 



Tentacles 12. Calcareous deposits consist exclusively of wheels, but these 

 are of two perfectly distinct kinds ; of these the smaller usually have 11 spokes 

 and 22 or more large inwardly directed teeth, as in Myriotrochus, while the 

 larger usually have 8-10 spokes, and alternating with them 8 outwardly pro- 

 jecting spines. 



This remarkable monotypic genus is still known only from the original 

 specimens of its describers. 



ACANTHOTROCHUS MIRABILIS. 



PLATE VIII, FIGS. 1-6. 



Acanthotrochus mirabilis Danielssen and Koren, 1879, p. 115; also 1882, p. 35, pi. vi. 

 LENGTH. 10-12 mm. 



COLOR. "Wanting; skin nearly transparent, "with glittering points." 

 DISTRIBUTION. Reported only from between Spitzbergen and Norway, in 

 1,203-2,030 m. (Danielssen and Koren). 



REMARKS. Nothing further is known of this interesting species beyond the 

 descriptions and figures of Danielssen and Koren. 



CONCLUDING REMARKS ON THE SYNAPTID^E. 



INTERRELATIONSHIPS. 



Having thus characterized the 21 genera and 88 species of Synaptidae known 

 to science at the present day, it may be of interest to attempt to show in graphic 

 form the relationship which they seem to have to each other. There is every 

 reason to believe that the ancestor of the group was a small 10-tentacled 

 apodous holothurian, and there is some reason for believing that the earliest 

 well-formed calcareous particles were wheel-shaped, for such particles are the 

 first to appear in the auricularia of Labidoplax (see Semon, '88). In the face 

 of such knowledge of echinoderm embryology as we have, we can hardly believe 

 that such a condition as occurs in Rhabdomolgus, the entire absence of calca- 



